The invention concerns a wettable cathode for an electrolytic cell for the electrolysis of a molten charge, in particular for the production of aluminum.
The use of wettable cathodes is known in connection with the production of metals by electrolytic reduction of a molten electrolyte. In the production of aluminum with electrolytic cells representing the state of the art, it is known that cathodes made of titanium boride, titanium carbide, pyrolitic graphite, boron carbide and other substances have been proposed, including mixtures of these substances which can be sintered together.
Cathodes which can be wet with aluminum, and which are not or only slightly soluble in aluminum, offer decisive advantages over conventional electrolytic cells in which the interpolar distance is approximately 6 to 6.5 cm. The aluminum deposited at the cathode flows over the cathode surface facing the anode surface even when the layer formed there is very thin. It is therefore possible to lead the liquid aluminum out of the gap between the anode and the cathode into a sump outside this gap. Because the layer of aluminum on the surface of the cathode is thin, there are no irregularities such as those which occur in conventional electrolytic reduction due to differences in thickness of the aluminum layer produced by electromagnetic and convection forces. Consequently, the interpolar distance can be reduced without a loss in yield i.e. significantly less energy is required per unit of metal electrolyzed.
In the U.S. Pat. No. 3,400,061 an electrolytic cell is proposed with wettable cathodes attached to the carbon floor of the cell. The cathode plates are slightly inclined, with respect to the horizontal, towards the middle of the cell. The gap between the anode and cathode i.e. the interpolar distance, is much smaller than in conventional cells with a carbon floor. This makes the circulation of electrolyte between anode and cathode more difficult. As the aluminum is deposited, the alumina content of the cryolite melt drops, and the anode effect can occur. Only a small part of the floor of the cell is available for collecting the liquid metal. In order that the tapping interval is not so short as to be uneconomical, the sump must be deep which in turn calls for extra insulation of the floor of the cell.
Furthermore, it should be noted that it is difficult to achieve proper electrical contact between the carbon floor and the wettable cathode plates with the mass used for this purpose. The electrical resistance of the floor of the cell is consequently increased. As with the normal electrolytic cells, the floor is made of electrically conductive carbonaceous material which provides poor thermal insulation.
Wettable cathodes are also employed in the process according to the German Pat. No. 26 56 579. In this publication the circulation of the cryolite melt is improved by anchoring the cathode elements in the electrically conductive floor and, in the area below the anode, having these project out of the aluminum gathered on the rest of the cell floor. In the case in question the cathode elements are tubes which are closed at one end, made of material which is wet by aluminum and completely filled with liquid aluminum. Gaps between the cathode elements, above the liquid aluminum, make the circulation of the electrolyte easier. The size of this gap is chosen such that there is no significant electrical contact between the anode and the liquid aluminum. The means of supplying electrical power to the cathode elements, described in the German patent, suffer from the disadvantages associated with power supply made through the carbon floor. The electrolyte flows in a whirlpool-like manner around the cathode element i.e. no direction in particular being preferred. Consequently, the alumina concentration is not distributed in the best possible manner.
One disadvantage of arrangements with wettable cathodes which have been tested in practice is that the cathode is anchored in the floor of the cell. For economic reasons therefore one must choose for the wettable cathode plates a material with a service life which is at least equal to or better than that of the lining of the cell. The use of a cheaper material with a shorter service life or simpler manufacturing technology means that with the failure of only a small part of the cathode element, for example due to mistakes in operation or manufacture, there is the risk of having to shut down the whole cell. The carbon floor with cast-in cathode bars is in fact extremely sensitive to flaws during manufacture.
The inventor set himself the task of developing a wettable cathode for a molten salt electrolytic cell, in particular for a cell for the production of aluminum, in which a considerable reduction in the interpolar distance is permitted, without disadvantageously affecting the circulation of the electrolyte and the collection of the deposited metal, the said wettable cathode to be such that it can be manufactured by straightforward technology from favorably priced material, without reducing the lifetime of the cell.